In geometry, a "normal" is a line that is drawn perpendicular to
a surface. The normal force is simply the reaction to a force pushing
on an object. For a ball resting on a table, the normal force would
be equal and opposite to the weight of the ball. The surface of
the table deforms slightly and produces a reaction force equal to
the force pressing the ball onto the surface. If the table were
tilted, the normal force would still be perpendicular to the table,
but it would become smaller.

The word "equilibrium,'' literally translated, means "equal balance."
The idea of equilibrium is used in many sciences, from ecology to
chemistry, and can be applied to both static and dynamic systems.
We have seen several examples of objects in equilibrium in our workshops
-- both objects at rest, like the ball supported by a hand, and
objects moving at constant speed, like the falling coffee filter.
Any time the net force is zero, we know that the forces acting on
the object are balanced. In the case of an object that could rotate,
we would also have to account for torques that would make it turn.

Paul's students are suspending various masses from springs and
rubber bands and measuring the stretch that the force of the weight
causes. In order to compare their data, the students need to plot
the force in Newtons and the stretch in meters. They have learned
that 1 kilogram weighs 9.8 Newtons and that 1000 grams equals 1
kilogram (1000 g = 1 kg). The students first change the mass they
recorded from grams to kilograms, then convert the mass to weight
so that they have force units. Finally, they convert their stretch
measurements from centimeters to meters (100 cm = 1 m) and plot
their graphs.